White light-emitting LED lamps have been practically employed as a back-light of a liquid crystal display panel or a lighting. The conventionally employed light-emitting LEDs are of a double color admixture type of which white light is produced by mixing a blue light released from a semiconductor light-emitting element by application with electric energy and a yellow light emitted by a yellow light-emitting phosphor when excited with the blue light from the semiconductor light-emitting element. However, there is a problem in the white light emitted by the white light-emitting LED lamp of the double color admixture type, in that the white light is not sufficiently pure. Therefore, there have been studied white light-emitting LED lamps of a triple color admixture type which give a white light by mixing a blue light, a green light and a red light. The blue light, green light and red light are all released respectively from a blue light-emitting phosphor, a green light-emitting phosphor and a red light-emitting phosphor by excitation with a light in the wavelength region of 350 to 430 nm which is released from a semiconductor light-emitting element by applying thereto electric energy.
The white light-emitting LED lamp generally comprises a light-emitting semiconductor element and a phosphor layer placed around the light-emitting semiconductor element. The phosphor layer comprises a transparent material and a phosphor dispersed therein. The transparent material generally is a thermoplastic resin such as a silicone resin. The phosphor layer is generally produced by heating a mixture of a phosphor and a transparent material until the mixture is softened, and then cooling the softened mixture until it is cured.
As a blue light-emitting phosphor, there is known a blue light-emitting silicate phosphor having an elemental formula of Sr3MgSi2O8 activated with a divalent Eu and a merwinite crystal structure, that is, a crystal structure equivalent to the merwinite (Ca3MgSi2O8). This blue light-emitting silicate phosphor is hereinafter referred to as SMS blue light-emitting phosphor. The SMS blue light-emitting phosphor has been studied for the use as a blue light-emitting material to be placed in a plasma display as well as in a white light-emitting LED of the three color mixture type, as is disclosed in JP 2009-280793A.
In more detail, JP 2009-280793 A describes the use of a blue light-emitting phosphor comprising a main (primary) crystal phase of M13MgSi2O8 (M1 is Sr and Ba) crystal comprising Eu and a secondary crystal phase of M12MgSi2O7.